[A] dental prosthesis that is borne by implants must fit very precisely on these implants, since germs may accumulate in potentially present interstices between implant and dental prosthesis, which may lead to inflammation (peri-implantitis). In the manufacturing of the dental prosthesis, there are some sources of error that are relevant to this:                In taking impressions and the production of a plaster model (master model) for the adaptation by a dental technician of a restoration to be created, systematic transfer errors arise, which turn out to be larger or smaller, depending upon the thoroughness of the treating dentist and upon the materials used.        Additional errors and inaccuracies may occur upon scanning in the master model for creating restorations by means of CAD/CAM. However, these errors can be minimized via the use of special scanners having especially high precision.        An alternative procedure, in which a virtual master model is created by means of an intra-oral scan, likewise causes inaccuracies—especially, due to the technical limitations of the available intra-oral scan technology.        
Nevertheless, for the creation of restorations that are respectively installed on a single implant, it can be ensured relatively well that the restoration precisely fits on the implant.
However, with multi-element, implant-borne restorations, i.e., restorations that are installed on more than one implant, these errors have severe effects. Namely, the precise relative position, and, especially, also the relative orientation, of the implants here depend upon this, so that no gaps can arise between the implants and the restorations. In such a case, the restoration must possibly even be completely recreated. Many dental laboratories have therefore moved to guaranteeing the fit only for the master model, and making no guarantee as to the fit in the mouth of the patient.